In a departure from the traditional semi-rigid rotor system design, Frank Robinson used a coning hinge on each blade when designing the R22 in the 1970s. When rotor blades produce lift (especially under high load or low rotor rpm) they flex upward (coning). Although some mistakenly refer to this as a flapping hinge, it is used for blade coning. Previous rotor systems used a fixed coning angle built into each blade. Robinson’s design allows the coning angle to vary according to different conditions such as rotor speed, acceleration, and weight.
Coning via blade bending places a high stress load at the blade’s root. The coning hinge relieves this stress, reducing the amount of reinforcing required and making for a lighter, easier to manufacture rotor blade. Additionally, the reduced bending of the rotor blade at its root allows the pitch-change axis to be better aligned with the blade’s centerline. This reduces the forces across the hub, pitch change bearings, and the rotor blades and, as such, decreases shake and feedback in the cyclic control.
Known as a tri-hinge rotor hub (the third hinge is called a teetering hinge and is typically the only hinge in a semi-rigid rotor system) Robinson was granted a patent for it in 1978. The same hub design (although larger in size) is used on the R44 and the new turbine-powered R66.